1. WD Blue 4TB Model WD40EZRZ
2. WD Blue SSHD 4TB Model WD40E31X
3. WD Black 4TB Model WD4003FZEX
4. WD Re 4TB Model WD4000FYYZ
1. Standard basic model with the lowest speed, durability and warranty, also the cheapest
2. For when you cant afford an SSD as your boot drive but otherwise the same as 1.
3. Faster than 1, more expansive than 1 with a slightly better warranty (I think)
4. Probably not any faster than 1 but is more durable and rated for 24/7 use, longer warranty
But all this stuff is on Western Digital's website, just saying.